1. Field of Art
This invention relates to a hydrophilized membrane of a porous hydrophobic material and a process for preparing such a membrane.
In recent years, porous hydrophobic membranes made of polymer materials, such as polyethylene, polypropylene, polytetrafluoroethylene and copolymers of an olefin monomer and a fluorinated olefin monomer, have been used for water treatment applications, since they are excellent in water resisting property and resistance to chemicals and attacks by bacteria. For instance, such a membrane is used in medical facilities to obtain germ-free water and also used in a semiconductor manufacturing factory to obtain water of high purity.
Although these hydrophobic membranes are porous, water is not allowed to pass through or permeate them, if not subjected to a high pressure. Therefore, such hydrophobic membranes under consideration need to be hydrophilized to allow water to permeate them.
2. Related Art Statements
Various proposals have hitherto been made to hydrophilize such a hydrophobic membrane, including a method of treating a hydrophobic membrane with corona discharge, and a process wherein a hydrophobic membrane is processed with a strong oxidizing agent. However, the micropore structure of the membrane can be seriously affected by the corona discharge, resulting in deleterious change in structure, in addition to the disadvantage that the membrane cannot be permanently hydrophilized by the corona discharge treatment. On the other hand, processing with a strong oxidizing agent has disadvantages that the matrix hydrophobic material is seriously deteriorated by the oxidizing agent and that the oxidizing agent impregnating deep into the micropores is hardly removed therefrom.
Japanese Patent Laid-Open Publication No. 59-501049 discloses a process wherein a porous hydrophobic membrane is hydrophilized by coating the membrane with a mixture of a carbohydrate and a non-ionic ester of an organic monocarboxylic acid, such as a monoester of sorbitan and capric acid, lauric acid, myristic acid, palmitic acid and/or oleic acid. Although the hydrophobic membrane is hydrophilized by this process, the membrane prepared therethrough has a strong odor to make it undesirable to use the membrane as a filter for a water purifier for drinking use. Moreover, the water passing through the membrane processed in accordance with this preceding proposal suffers foaming or bubbling due to entrainment of the processing agent.
The method of hydrophilizing a porous hydrophobic membrane, which has been recommended as the most reliable and preferable up to date, involves the step of passing a liquid soluble in water and having a low surface tension, such as ethanol, through the pores of the membrane followed by replacing the liquid by water. With this method, so long as water is present in the pores of the hydrophobic membrane, water is allowed to pass through the pores under a relatively small pressure. However, once the water is removed from the pores so that the pores become dry either partially or entirely and are filled with air, the hydrophilized hydrophobic membrane is rendered hydrophobic again, and water is not allowed to pass through the pores unless it is not subjected to an extremely high pressure. Thus, when the hydrophobic membrane is hydrophilized with this conventional method, it must be always kept in wet condition to maintain the hydrophilic nature, leading to cumbersome maintenance problem.